1. Field of the Invention
The present invention relates to an electronic watermark generating method and electronic watermark generating program for an irreversible compression image such as a JPEG image.
2. Description of the Related Art
Conventionally, in the case where to-be-embedded information for generating an electronic watermark with respect to an irreversible compression image such as a JPEG image is embedded, and the embedded information is extracted from the irreversible compression image, such an embedding and extracting operation has been carried out for image data at a stage at which an irreversible process has completely terminated during image compression processing. For example, in the case where an electronic watermark is generated with respect to a JPEG image, embedding of to-be-embedded information is carried out with respect to a quantized coefficient before an encoding process is carried out after a quantizing process in a course of generating a JPEG image from an RGB image, or alternatively, before de-quantizing is carried out after the JPEG image has been subjected to a decoding process in the case of using a JPEG image (Japanese Patent No. 3098513, for example).
However, an electronic watermark generated with respect to a conventional irreversible compression image is not sufficient in durability. Therefore, in the case where, for example, an RGB image is restored by temporarily carrying out decoding, de-quantizing, inverse quadrature transform, block re-combining, and color conversion in order to produce an irreversible compression image having an electronic watermark embedded therein as one having a compression rate according to equipment specification, and then, the restored RGB image is produced as an irreversible compression image having a different compression rate by carrying out color conversion, dividing into a plurality of blocks, quadrature transform, quantizing, and encoding again, there has been a high possibility that information embedded as an electronic watermark cannot be extracted.